A major goal of this proposal is to understand the role that cytotoxic T lymphocyte (CTL) escape plays in HIV transmission and disease pathogenesis. An understanding of the impact of CTL escape mutations on viral evolution and fitness is only beginning to unfold and the studies proposed in this application represent a unique opportunity to examine how HIV adapts as it moves from one HLA environment to another. These studies focus on linked heterosexual transmission pairs to extensively characterize CTL escape mutations in the setting of shared and disparate HLA class I alleles and to follow these longitudinally after HIV-1 transmission, possible reasons for delayed or non-transmission will also be examined in an equivalent group of discordant couples. Specifically we will: 1.Perform a cross-sectional analysis of HIV adaptation to HLA-class I restricted immune responses at a population and individual level to determine if HIV-1 polymorphisms associated with HLA alleles cluster within known HLA class I specific CD8+ CTL epitopes and if we can identify novel epitopes that are specific to African HLA alleles. The epitopes and escape mutations will be validated biologically. These approaches will be extended to determine the relative contribution of HLA allele frequency and viral replicative fitness to fixation of escape mutation in the population consensus sequence, and analyses of class II MHC and Kir allele contributions to immune control of HIV. 2. In 150 couples where we have documented recent transmission, we will determine the kinetics of CTL epitope reversion and escape as the virus adapts to moving from the HLA class I environment of the donor to that of the recipient. We will determine the biological consequences of persistence, escape and reversion in epitope-specific gag and nef sequences by analysis of viral load and in vitro fitness. 3. In approximately 40 high-risk non-transmitting couples we will determine if the long-term non-transmitting partners display a bias in escape signatures that may contribute to lack of transmission. These studies will further allow the application and development of models that address the impact of CTL escape mutations on viral load and viral fitness and their relevance to vaccine development.